Flexible wiring for low temperature applications

ABSTRACT

The subject matter of the present disclosure may be embodied in devices, such as flexible wiring, that include: an elongated flexible substrate; multiple electrically conductive traces arranged in an array on a first side of the elongated flexible substrate; and an electromagnetic shielding layer on a second side of the elongated flexible substrate, the second side being opposite the first side, in which the elongated flexible substrate includes a fold region between a first electronically conductive trace and a second electrically conductive trace such that the electromagnetic shielding layer provides electromagnetic shielding between the first electronically conductive trace and the second electrically conductive trace.

TECHNICAL FIELD

The present disclosure relates to flexible wiring for low-temperatureapplications, such as quantum processors using superconducting qubits.

BACKGROUND

Quantum computing is a relatively new computing method that takesadvantage of quantum effects, such as superposition of basis states andentanglement to perform certain computations more efficiently than aclassical digital computer. In contrast to a digital computer, whichstores and manipulates information in the form of bits (e.g., a “1” or“0”), quantum computing systems can manipulate information using qubits.A qubit can refer to a quantum device that enables the superposition ofmultiple states (e.g., data in both the “0” and “1” state) and/or to thesuperposition of data, itself, in the multiple states. In accordancewith conventional terminology, the superposition of a “0” and “1” statein a quantum system may be represented, e.g., as α|0>+β|1>. The “0” and“1” states of a digital computer are analogous to the |0> and |1> basisstates, respectively of a qubit. The value |α|² represents theprobability that a qubit is in |0> state, whereas the value |β|²represents the probability that a qubit is in the |1> basis state.

SUMMARY

In general, in some aspects, the subject matter of the presentdisclosure may be embodied in devices, such as flexible wiring, thatinclude: an elongated flexible substrate; multiple electricallyconductive traces arranged in an array on a first side of the elongatedflexible substrate; and an electromagnetic shielding layer on a secondside of the elongated flexible substrate, the second side being oppositethe first side, in which the elongated flexible substrate includes afold region between a first electronically conductive trace and a secondelectrically conductive trace such that the electromagnetic shieldinglayer provides electromagnetic shielding between the firstelectronically conductive trace and the second electrically conductivetrace.

Implementations of the devices may include one or more of the followingfeatures. For example, in some implementations, the fold region includesa raised band in the flexible substrate, and a length of the elongatedraised band extends parallel to a length of the first electricallyconductive trace and the second electrically conductive trace.

In some implementations, the flexible wiring includes a first elongatedgroove in the fold region, and a length of the first elongated grooveruns parallel to a length of the first electrically conductive trace anda length of the second electrically conductive trace.

The first elongated groove may extend into the first side or into thesecond side of the elongated flexible substrate. The flexible wiring mayinclude a second elongated groove in the fold region, where a length ofthe second elongated groove runs parallel to the length of the firstelectrically conductive trace and the length of the second electricallyconductive trace, and where the first elongated groove is on the firstside of the substrate and the second elongated groove is on the secondside of the substrate. The first elongated groove may extend into theelectromagnetic shielding layer. The first elongated groove may extendinto the elongated flexible substrate.

In some implementations, at least one electrically conductive trace ofthe multiple electrically conductive traces includes a bi-layer, thebi-layer having a superconductor layer and a metal layer on thesuperconductor layer. The superconductor layer may include niobium orNbTi. The metal layer may include copper or a copper alloy.

In some implementations, the electromagnetic shielding layer includes abi-layer, the bi-layer including a superconductor layer and a metallayer on the superconductor layer. The superconductor layer may includeniobium or NbTi. The metal layer may include copper or a copper alloy.

In some implementations, the electromagnetic shielding layer includesmultiple microstrips having lengths oriented orthogonally with respectto the lengths of the plurality of electrically conductive traces.

In general, in some other aspects, the subject matter of the presentdisclosure may be embodied in devices, such as flexible wiringincluding: a first elongated flexible layer; a second elongated flexiblelayer bonded to the first elongated flexible layer; multipleelectrically conductive traces arranged at a bond interface between thefirst elongated flexible layer and the second elongated flexible layer;a first electromagnetic shielding layer on a principal surface of thefirst elongated flexible layer; a second electromagnetic shielding layeron a principal surface of the second elongated flexible layer; and a viaextending through the first elongated flexible layer, wherein the viaincludes a superconductor via contact.

Implementations of the flexible wiring may include one or more of thefollowing features. For example, in some implementations, the viaincludes an adhesive layer, the superconductor via contact being formedon the adhesive layer.

In some implementations, the via extends from the first electromagneticshielding layer to at least one electrically conductive trace of theplurality of electrically conductive traces, and the superconductor viacontact is connected to the first electromagnetic shielding layer andthe at least one electrically conductive trace.

In some implementations, the via extends from the first electromagneticshielding layer to the second electromagnetic shielding layer, and thesuperconductor via contact is connected to the first electromagneticshielding layer and the at least one electrically conductive trace.

In general, in other aspects, the subject matter of the presentdisclosure may be embodied in devices including: a first flexible wiringcomprising a first elongated flexible substrate, a first multiple ofelectrically conductive traces arranged in an array on a first side ofthe first elongated flexible substrate, and a first electromagneticshielding layer on a second side of the first elongated flexiblesubstrate, the second side of the first elongated flexible substratebeing opposite the first side of the first elongated flexible substrate;a second flexible wiring including a second elongated flexiblesubstrate, a second multiple of electrically conductive traces arrangedin an array on a first side of the second elongated flexible substrate,and a second electromagnetic shielding layer on a second side of thesecond elongated flexible substrate, the second side of the secondelongated flexible substrate being opposite the first side of the secondelongated flexible substrate, in which the first flexible wiring iscoupled to the second flexible wiring through a butt joint.

Implementations of the devices may include one or more of the followingfeatures. For example, in some implementations, the butt joint includesa wire bond that connects a first electrically conductive trace from thefirst multiple of electrically conductive traces to a first electricallyconductive trace from the second multiple of electrically conductivetraces.

In some implementations, the butt joint includes a solder bridge thatconnects a first electrically conductive trace from the first multipleof electrically conductive traces to a first electrically conductivetrace from the second multiple of electrically conductive traces.

In some implementations, the devices include a metal block secured toand in thermal contact with the first electromagnetic shielding layerand to the second electromagnetic shielding layer.

In general, in other aspects, the subject matter of the presentdisclosure may be embodied in devices including: a first flexible wiringincluding a first elongated flexible substrate, a first multiple ofelectrically conductive traces arranged at a bond interface within thefirst elongated flexible substrate, a first electromagnetic shieldinglayer on a first principal surface of the first elongated flexiblesubstrate, and a second electromagnetic shielding layer on a secondprincipal surface of the first elongated flexible substrate; a secondflexible wiring including a second elongated flexible substrate, asecond multiple of electrically conductive traces arranged at a bondinterface within the second elongated flexible substrate, a thirdelectromagnetic shielding layer on a first principal surface of thesecond elongated flexible substrate, and a fourth electromagneticshielding layer on a second principal surface of the second elongatedflexible substrate, in which the first flexible wiring is electricallycoupled to the second flexible wiring through a butt joint.

Implementations of the devices may include one or more of the followingfeatures. For example, in some implementations, the first elongatedflexible substrate includes a first cavity through which a firstelectrically conductive trace of the first multiple of electricallyconductive traces is exposed, and the second elongated flexiblesubstrate comprises a second cavity through a first electricallyconductive trace of the second multiple of electrically conductivetraces is exposed. The butt joint may include a wire bond that connectsthe exposed first electrically conductive trace of the first multiple ofelectrically conductive traces to the exposed first electricallyconductive trace of the second multiple of electrically conductivetraces.

In some implementations, the butt joint includes a solder bridge thatconnects the exposed first electrically conductive trace of the firstmultiple of electrically conductive traces to the exposed firstelectrically conductive trace of the second multiple of electricallyconductive traces.

In some implementations, the devices further include a first metal blocksecured to and in thermal contact with the first electromagneticshielding layer and to the third electromagnetic shielding layer. Thedevices may further include a second metal block secured to and inthermal contact with the second electromagnetic shielding layer and tothe fourth electromagnetic shielding layer.

In general, in other aspects, the subject matter of the presentdisclosure may be embodied in systems including: a cryostat including afirst stage configured to be held within a first temperature range; aquantum information processing system within the first stage; andflexible wiring within the first stage and coupled to the quantuminformation processing system, in which the flexible wiring includes anelongated flexible substrate, a multiple of electrically conductivetraces arranged in an array on a first side of the elongated flexiblesubstrate, and an electromagnetic shielding layer on a second side ofthe elongated flexible substrate, the second side being opposite thefirst side, in which the elongated flexible substrate includes a foldregion between a first electronically conductive trace and a secondelectrically conductive trace such that the electromagnetic shieldinglayer provides electromagnetic shielding between the firstelectronically conductive trace and the second electrically conductivetrace.

In general, in other aspects, the subject matter of the presentdisclosure may be embodied in systems including: a cryostat including afirst stage configured to be held within a first temperature range; aquantum information processing system within the first stage; andflexible wiring within the first stage and coupled to the quantuminformation processing system, in which the flexible wiring includes afirst elongated flexible layer; a second elongated flexible layer bondedto the first elongated flexible layer; multiple electrically conductivetraces arranged at a bond interface between the first elongated flexiblelayer and the second elongated flexible layer; a first electromagneticshielding layer on a principal surface of the first elongated flexiblelayer; a second electromagnetic shielding layer on a principal surfaceof the second elongated flexible layer; and a via extending through thefirst elongated flexible layer, in which the via includes asuperconductor via contact.

Particular implementations of the subject matter described here canrealize one or more of the following advantages. For example, in someimplementations, folded regions of the flexible wiring provideelectromagnetic shielding between signal traces. The shielding canreduce crosstalk without requiring the formation of via holes within thesubstrate. In some implementations, when vias are provided within thesubstrate, the via may be filled with a superconducting material (e.g.,niobium) that allows for improved signal integrity and a reduction incrosstalk. Additionally, because material that is superconducting doesnot exhibit a DC resistance, the via metal will not lead to resistiveheating. In some implementations, the flexible wiring allows asubstantial increase in the number and density of wires that can beconnected to a device (e.g., a quantum information processing system)contained within a cryostat relative to apparatuses that use co-axialcables. Additionally, by using flexible wiring in place of co-axialcables, the space otherwise required by co-axial cables may be freed upfor purposes other than providing electrical connections. In someimplementations, the flexible wiring utilizes material, such as copper,a copper alloy (e.g., brass), or a superconductor (e.g., NbTi), thatoffers a relatively low thermal conductivity and therefore low heatload. Furthermore, in some implementations, the fabrication costsassociated with flexible wiring may be lower on a per wire basis thanapparatuses that rely on co-axial cables. In some implementations, theflexible wiring may be bonded to other flexible wiring using a buttjoint in place of co-axial cable connectors. By using a butt joint inplace of co-axial cable connectors, a large number of connections may beestablished. Additionally, using a butt joint may free up space withinthe cryostat that would otherwise be used by the co-axial cableconnectors. Moreover, in some implementations, butt joints provide alower cost fabrication technique relative to soldering circuit boardstogether, especially when large numbers of connections need to be made.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features andadvantages will be apparent from the description, the drawings, and theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic that illustrates an example of a refrigerationsystem for cooling quantum information processing systems.

FIG. 2 is a schematic that illustrates an example of flexible wiring.

FIG. 3 is a schematic that illustrates an example of flexible wiring.

FIG. 4 is a schematic that illustrates an example of flexible wiring.

FIG. 5 is a schematic that illustrates an example of flexible wiring.

FIG. 6 is a schematic that illustrates an example of a modified buttjoint bond for flexible wiring.

FIG. 7A is a schematic that illustrates an example of a modified buttjoint bond for flexible wiring.

FIG. 7B illustrates a side view of flexible wiring shown in FIG. 7A.

DETAILED DESCRIPTION

Quantum computing entails coherently processing quantum informationstored in the quantum bits (qubits) of a quantum computer.Superconducting quantum computing is a promising implementation ofsolid-state quantum computing technology in which quantum informationprocessing systems are formed, in part, from superconducting materials.To operate quantum information processing systems that employsolid-state quantum computing technology, such as superconductingqubits, the systems are maintained at extremely low temperatures, e.g.,in the 10s of mK. The extreme cooling of the systems keepssuperconducting materials below their critical temperature and helpsavoid unwanted state transitions. To maintain such low temperatures, thequantum information processing systems may be operated within acryostat, such as a dilution refrigerator. In some implementations, thelimited cooling capacities of such cryostats requires that controlsignals be generated in higher-temperature environments, where muchgreater cooling capacity is available and dissipative circuits are lesslikely to disrupt qubits within the quantum information processingsystem. Control signals may be transmitted to the quantum informationprocessing system using shielded impedance-controlled GHz capabletransmission lines, such as coaxial cables.

It is expected that the number of qubits utilized in quantum informationprocessing systems will increase significantly (e.g., tens of thousands,hundreds of thousands, millions or more) in the near future. Withincreasing numbers of qubits, the number of transmission lines (e.g.,control and data lines) required for driving the qubits and for readingoutput from operations performed by the quantum information processingsystems also are likely to substantially increase.

The present disclosure is directed to wiring for low-temperatureapplications, such superconducting quantum information processingsystems, in which the wiring allows, in certain implementations,substantial increases in transmission line density, while maintaininglow cross-talk among transmission lines, as well as low heat loads.Additionally, the devices and methods disclosed herein may provide, incertain implementations, low cost alternatives to bulky transmissionlines such as coaxial cables.

FIG. 1 is a schematic that illustrates an example of a refrigerationsystem 100 for cooling quantum information processing systems. Theexemplary refrigeration system 100 includes a cryostat 102 in which aquantum information processing system 110 may be contained. The cryostat102 cools the ambient environment surrounding the quantum informationprocessing system 110 to a temperature that is suitable for performingoperations with the system 110. For example, in implementations in whichthe quantum information processing system 110 includes a quantumprocessor having superconducting qubits, the cryostat 102 may cool theambient environment surrounding the quantum processor to temperaturesbelow the critical temperature of the superconducting material, e.g.,down to about 20 mK, or about 10 mK. Examples of superconductingmaterials that may be used in superconducting quantum informationprocessing systems include Al (Tc=1.2 K), In (Tc=3.4 K) and Nb (Tc=9.3K). The cryostat 102 may be cooled using liquid or gas cryogens, such ashelium and nitrogen, or cooled with a closed cycle cryocooler usinghelium gas. In some cases, the circuit elements of the quantuminformation processing system 110 operate at microwave frequencies(e.g., frequencies in the range of about 300 MHz to about 100 GHz, suchas between about 300 MHz and 10 GHz). Accordingly, the cryostat 102 mayinclude external and internal electromagnetic shielding to blockinterference with the quantum information processing system 110.

In some implementations, a cryostat includes multiple thermally isolatedstages that span a large temperature difference (e.g., different stagesof a dilution refrigerator). For instance, the exemplary cryostat 100includes multiple stages 101, 103, and 105. The first stage 101 may bemaintained at a first temperature range T1, whereas the second stage110306 may be maintained at a second temperature range T2 that is lowerthan the first temperature T1, and the third stage 105 may be maintainedat a third temperature range T3 that is lower than the secondtemperature T2. For example, the third temperature range T3 may be at orlower than the critical temperature Tc for the superconducting materialsthat are used in the quantum information processing system 110, e.g.,T2≈10-20 mK. In contrast, the second stage 103 may be maintained at ahigher temperature than the third stage 105. For example, the secondstage 103 may be maintained within a temperature range T2 that is lessthan 3 K and greater than 20 mK. The first stage 101 may be maintainedwithin a temperature range that is higher than the second stage 103. Forexample, the first second stage 101 may be maintained within atemperature range T1 that is less than 300 K and greater than 3 K.Though only three stages are shown in the example of FIG. 1, thecryostat can include additional stages at different temperature levels.For instance, in some cases, the cryostat may include fourth and fifthstages held within a fourth temperature range T4 and a fifth temperaturerange T5, respectively. Each temperature stage of the cryostat typicallyspans, e.g., a few cm in length to the next temperature stage.

Each stage within the cryostat 100 may be separated by a boundary 104,106. The boundaries 104, 106 may include thermal sinks held at aconstant temperature. The stages of the cryostat 102 are operated undera vacuum environment. For example, the first stage 101, the second stage103, and the third stage 105 may be operated under a vacuum basepressure of about 1×10⁻⁷ Torr or less. The quantum informationprocessing system 110 may include a substrate (e.g., a dielectricsubstrate such as silicon or sapphire) on which are formed quantuminformation processing devices, such as qubits. The qubits may becouplable to one another so that, during operation of the system 110,the qubits perform useful computations. In addition to qubits, thequantum information processing system 110 may include other components,such as measurement readout devices, coupler devices for coupling thequbits, and control devices for driving and tuning the qubits. Thequantum information processing system 110 may be positioned and/or fixedto a sample mount 112 within the third stage 105.

To control and to read data from the quantum information processingsystem 110, the quantum information processing system 110 may be coupledto control electronics 150 arranged outside of the cryostat 102. In theexample shown in FIG. 1, the control electronics 150 are coupled to thequantum information processing system 110 within the cryostat 102 usingflexible wiring 114, 116. Signals generated by the control electronics150 or by the quantum information processing system 110 are transmittedover the flexible wiring 114, 116. The flexible wiring 114, 116includes, for example, multiple electrically conductive wires on orwithin an elongated flexible substrate. The flexible wiring 114, 116 mayinclude electromagnetic shielding to protect the wires from signalinterference. Additionally, the wires within the wiring 114, 116 may beimpedance matched to the quantum information processing system 110 andthe control electronics 150 to reduce signal reflections from loads.

Each flexible wiring 114, 116 may include multiple individual wires. Theindividual wires may extend along the length (the long dimension) of theflexible wiring 114, 116, and may be arranged in an array (e.g., thewires may extend in parallel along the length of the flexible wiring114, 116). The total number of wires within or on a flexible wiring mayvary. For example, each flexible wiring 114, 116 may include 10 or morewires, 20 or more wires, 30 or more wires, 50 or more wires, 100 or morewires, or 200 or more wires. Other numbers of wires may be used withineach flexible wiring 114, 116 as well. Each flexible wiring may becoupled to another flexible wiring so that data and control signals maybe transmitted from one flexible wiring to another. For example,flexible wiring 114 may be coupled to flexible wiring 116. In someimplementations, a first set of at least two flexible wiring are coupledto a second set of at least two flexible wiring, respectively. Forexample, a first set of 5, 10, 15, 20 or more flexible wiring may becoupled to a second set of 5, 10, 15, 20 or more flexible wiring,respectively. Other numbers of flexible wiring may be coupled together.For the first and/or second set, the flexible wiring within the set maybe stacked directly on one another or, alternatively, individualflexible wiring within the stack may be separated from one another usinga spacer (e.g., a 2-10 mm spacer). An advantage of forming multiplewires within flexible wiring, and/or using multiple flexible wiring, isthat, in some implementations, the smaller footprint and greater wiredensity of the flexible wiring allows the total number of connectionsbetween the quantum information processing system and the controlelectronics to be greatly increased compared to apparatuses that rely oncoaxial cables. In some implementations, short sections of the flexiblewiring 114, 116 are clamped at the boundaries 104, 106 or elsewherewithin the cryostat 100 to thermally sink the flexible wiring 114, 116.For instance, wiring 116 may be clamped to a heat sink held at atemperature of 3 K at boundary 104. Similarly, wiring 114 may be clampedto a heat sink held at a temperature of 20 mK at boundary 106. Incontrast, the distance between boundaries where the wiring 114, 116 ismuch longer than the clamping length to decrease the flow of thermalenergy.

FIG. 2 is a schematic that illustrates an example of flexible wiringthat may be used for low temperature applications, including, e.g., forcoupling to quantum information processing systems within cryostats,such as cryostat 102. A plan-view of a top surface of a flexible wiring200 and a cross-section view of the flexible wiring 200 along line A-Aare shown in FIG. 2. As shown in the plan-view, flexible wiring 200includes an elongated flexible substrate 202. The flexible wiring 200also includes multiple electrically conductive traces 204 arranged on aprincipal surface (e.g., the top surface or side) of the elongatedflexible substrate 202. Each conductive trace 204 corresponds to anindividual wire and the multiple traces 204 may be arranged in an array.For example, the conductive traces 204 may arranged in parallel, withtheir long dimension (e.g., their length) extending along the longdimension (length) of the elongated flexible substrate 202. The spacingbetween adjacent traces 204 may be the same for each pair of adjacenttraces 204.

The elongated flexible substrate 202 may be formed from a flexibleplastic ribbon, such as a polyimide ribbon. Examples of materials thatcan be used for the elongated flexible substrate 202 include, e.g., poly(4,4′-oxydiphenylene-pyromellitimide) (also referred to as Kapton®). Thethickness of the elongated flexible substrate 202 may be, e.g., betweenabout 10 μm to about 500 μm, including thicknesses such as 20 μm, 50 μm,75 μm, and 100 μm, among others. The width of the elongated flexiblesubstrate 202 may be, e.g., between about 1 mm to about 30 mm, includingwidths such as 10 mm, 15 mm, and 20 mm, among others. The length of theelongated flexible substrate 202 may be at least as long as is necessaryto provide coupling between devices, systems and/or other wiring.

The conductive traces 204 include thin film materials that may bepatterned on the elongated flexible substrate 202. The conductive traces204 may include, e.g., a single layer of material or a bi-layer ofmaterial. The material that may be used to form the conductive traces204 may include superconducting material and/or metals that are notsuperconducting. Examples of materials that may be used to form theconductive traces 204 includes, e.g., copper, copper alloys(cupronickels, brass, bronze), aluminum, indium, NbTi, NbTi alloys,and/or niobium. Because in some cases, copper may have a thermalconductivity that is too high, and would otherwise lead to higherthermal transport, it may be advantageous to use copper alloys having alower thermal conductivity. This helps to decrease the lower thermalpower load, such that low temperatures required for operation of thequantum information processing system 110 may be maintained by thecryostat. In the case of a bi-layer trace, the trace 204 may include afirst layer formed on and in contact with the elongated flexiblesubstrate 202 and a second layer formed on and in contact with the firstlayer. The first layer of the bi-layer trace may include, e.g. asuperconducting material such as niobium, whereas the second layer ofthe bi-layer trace may include, e.g., a non-superconducting materialsuch as copper or a copper alloy. To improve adhesion of the metal orsuperconductor to the surface of the substrate 202, e.g., in the case ofa polyimide substrate, the substrate 202 may be ion milled.Alternatively, the first layer of the bi-layer trace may include anon-superconducting material such as copper and the second layer of thebi-layer trace may include a superconducting material such as niobium oraluminum. In some implementations, the material used to form theconductive traces may depend on where the flexible wiring is used in thecryostat. For example, in the lowest temperature regions (e.g., wherethe quantum information processing systems may be located) such asbetween 3 K to 10 mK, the flexible wiring may be formed using materialshaving low loss tangent and low thermal transport. In such cases, theconductive traces may be formed from superconductors, such as niobium.In higher temperature regions of the cryostat (e.g., where the wiringtransitions from low temperature to room temperature) such as fortemperatures above 3 K, the flexible wiring may be formed from materialswith are not superconducting but have lower thermal transport, such ascopper alloys, though high-temperature superconductors (e.g., Nb) alsomay be used. Additionally, in some implementations, the material formingthe traces may be selected for its role in providing solder connections.For example, copper may be used in regions where wire bonds or othersolder bonds are required.

The length of the conductive traces 204 may be as long as the length ofthe elongated flexible substrate 202. The width of each conductive trace204 may be, e.g., between about 1 μm to about 250 μm, including widthssuch as 5 μm, 10 μm, 20 μm, 30 μm, 50 μm, or 100 μm, among others. Insome implementations, the widths of the conductive traces are selectedto provide a predetermined impedance, e.g., a 50 ohm impedance, or a 75ohm impedance, in order to reduce signal reflections from a load. Thethickness of each conductive trace 204 may be, e.g., between about 10 nmto about 100 μm, including thicknesses such as 50 nm, 100 nm, 250 nm,500 nm, 750 nm, 1 μm, 5 μm, 10 μm, 20 μm, 50 μm, among others. In thecase of bi-layer conductive traces, each layer may have the same ordifferent thicknesses. For example, in some implementations, the firstlayer has a thickness of 2 μm, whereas the second layer has a thicknessof 5 μm. Alternatively, in some cases, the first layer has a thicknessof 20 μm, whereas the second layer has a thickness of 5 μm. Theconductive traces 204 may be separated by a constant or variable pitch.For example, in some implementations, the pitch between adjacentconductive traces 204 is between about 1 μm to about 1 mm, includingpitches such as 5 μm, 10 μm, 50 μm, 100 μm, 250 μm, 500 μm, or 750 μm,among others. The conductive traces 204 may be formed on the elongatedflexible substrate 202 using integrated chip (IC) fabrication techniquessuch as deposition (e.g., sputtering and vapor deposition), etching,and/or lift-off techniques.

As shown in the cross-section view A-A of FIG. 2, the flexible wiring200 includes an electrically conductive layer 208 on a second principalsurface/bottom side of the elongated flexible substrate 202, in whichthe second principal surface is opposite to that of the first principalsurface/top side. The electrically conductive layer 208 can be anelectromagnetic shielding layer for shielding the conductive traces 204from crosstalk. To allow the electromagnetic shielding layer 208 toprovide shielding between traces 204, the flexible substrate 202includes fold regions 206. Fold regions 206 includes regions of theflexible substrate 202 in which the substrate 202 has been folded suchthat an elongated raised band is provided. The elongated raised band ofthe fold region 206 may have a length that extends between and alongsidethe conductive traces 204. For example, the elongated raised band of thefold region 206 may extend parallel to the conductive traces 204 withinthe space between adjacent conductive traces 204. The raised band may beformed by folding the flexible substrate back on itself in a mannersimilar to a pleat. In some implementations, the peak or apex of theraised band extends above the top surfaces of the electricallyconductive traces 204 (e.g., the surfaces of the traces 204 facing awayfrom the substrate 202). With the substrate 202 folded in this manner,the electromagnetic shielding layer 208 in the fold region 206 createsan elongated arc that serves as a wall extending between adjacent traces204. The span of the arc of each fold region 206 is depicted by twoparallel dashed lines in the plan view of FIG. 2. Furthermore, the peakor apex of the elongated arc within the fold region 206 may extend abovethe top surface surfaces of the electrically conductive traces 204. As aresult, the shielding layer 208 within the fold region 206 provides anelectromagnetic barrier between adjacent traces to shield againstcrosstalk between the traces. As shown in the cross-section view of FIG.2, the fold regions 206 appear as raised fins and give the flexiblewiring an accordion-like shape. The fold regions 206 shown in FIG. 2include portions of the first principal surface of the flexiblesubstrate that do not have the conductive traces 204 formed on them. Inother implementations, the fold regions 206 may include portions of thefirst principal surface of the substrate 202 on which the conductivetraces 204 are formed. An advantage of introducing the fold regions 206is that shielding between traces can be provided without the need toprovide external shielding to the wires or to form shielding within thesubstrate 202 as might be needed for a stripline design. In someimplementations, the fold regions 206 may provide an order of 20-60 dBor higher reduction in crosstalk between adjacent conductive tracesrelative to a flexible wiring having the conductive traces formed on afirst principal side without the fold regions.

To hold the fold regions in place so that the substrate does not returnto its initial flattened state, the substrate and/or electromagneticshield layer 208 can be modified to introduce mechanical stress thathelps to hold the fold shape. FIG. 3 is a schematic that illustrates anexample of flexible wiring 250 that includes groove regions to introducemechanical stress for holding fold regions in place. Like wiring 200,flexible wiring 250 includes an elongated flexible substrate 202,electrically conductive traces 204 on a first principal side of thesubstrate 202, and an electromagnetic shielding layer 208 on a secondprincipal side of the substrate 202. The various parameters relating tomaterials and dimensions discussed herein with respect to wiring 200also may apply to wiring 250. A difference between wiring 250 (shown inthe plan view and cross-section view along line A-A in FIG. 3) and thewiring 200 in FIG. 2 is that the wiring 250 is shown in a flattenedstate to aid illustration of the grooves formed in the fold regions.Upon folding the substrate 202 to provide fold regions, the grooves mayprovide mechanical stress that holds the fold regions in place.

In some implementations, the grooves, e.g., grooves 210, are formedwithin the first principal surface of the substrate 202. A length of thegroove 210 extends parallel (e.g., long the X-direction in FIG. 3) to alength of the one or more adjacent conductive traces 204. The groove 210may have various different depths into the substrate 202. For example,the groove depth may range between about 1 μm to about 500 μm, such as10 μm, 20 μm, 50 μm, 70 μm, 100 μm, 200 μm, 250 μm, 300 μm, or 400 μm,among other depths. The groove 210 may have various different widths.For example, the groove width may range between about 10 μm to about 1mm, such as 20 μm, 50 μm, 100 μm, 250 μm, 500 μm, or 750 μm, among otherwidths. The groove 210 may extend the entire length of the elongatedflexible substrate 202 or may extend to a length that is shorter thanthe entire length of the elongated flexible substrate. In the example ofFIG. 3, each groove 210 is shown as a single continuous extendingbetween adjacent conductive traces 204. In other implementations,multiple separate individual grooves may be formed between theconductive traces 204, whether arranged in a single line or in a seriesof lines (e.g., a 2-dimensional array).

In some implementations, the grooves, e.g., grooves 212, are formedwithin the second principal surface of the substrate 202. The samevariation in groove depth, width length, and arrangement as explainedherein with respect to grooves 210 may apply to grooves 212. Theelectromagnetic shielding layer 28 may coat the grooves formed withinthe second principal surface of the flexible substrate 202. As shown inthe cross-section view along line A-A of FIG. 3, the grooves 212 may bepositioned between adjacent conductive traces 204, e.g., along theY-axis. In some implementations, as shown in FIG. 3, the grooves 212 areformed within the electromagnetic shielding layer 208 instead of thesecond principal surface of the substrate 202. That is, theelectromagnetic shielding layer 208 may be patterned (e.g., throughphotolithography and etching or lift-off processes) in such a way thatthe openings are formed within the electromagnetic shielding layer aloneand not within the substrate 202. The groove depth may extend entirelythrough the electromagnetic shielding layer 208 or partially through theelectromagnetic shielding layer 208. In some implementations, the grooveextends through the electromagnetic shielding layer 208 and into thesecond principal surface of the flexible substrate 202 as depicted bygroove 214 bound by dashed lines in the cross-section view of FIG. 3.

The grooves may be formed in the flexible substrate 202 usingphoto-processing techniques (e.g., spin-coating resist on the substrate202, exposing and developing a pattern in the resist, and etchingexposed regions of the substrate 202 to form grooves). In otherimplementations, the grooves may be formed user laser processing (e.g.,polyimide laser drill technology).

In some implementations, the fold regions of flexible wiring may be heldin place by arranging the electromagnetic shielding layer in multiplestrips that extend across the second principal surface of the elongatedflexible substrate. The strips may be used instead of or in addition togrooves as described herein. For example, FIG. 4 is a schematic thatillustrates an example of flexible wiring 300. In particular, FIG. 4includes a plan-view of the second principal surface of a flexiblewiring substrate 302 on which electromagnetic shielding 308 is formed,and a cross-section view through substrate 302 at line A-A. In the planview of FIG. 4, the position, boundaries and arrangement of theconductive traces 304 formed on the first principal surface of thesubstrate 302 are depicted using dashed lines. The flexible wiring 300is shown in its flattened state, i.e., without the fold regions yetformed.

As shown in FIG. 4, the electromagnetic shielding layer 308 is arrangedin multiple separate strips that have a length extending along theY-direction. Upon folding the substrate 302 to provide fold regions, theshielding layer strips 308 may provide mechanical stress that holds thefold regions in place. As further shown in the plan view of FIG. 4, thelengths of strips 308 extend along a direction (Y-direction) that isorthogonal to a direction (X-direction) along which the lengths of theconductive traces 304 extend. An advantage of using separate strips toform the electromagnetic shielding layer 308 is that the lower overallthermal transport can be achieved since less thermally conductivematerial is being used to provide the shielding layer.

Alternatively, in some implementations, the strips may be formed inaddition to a grounding layer rather than as the grounding layer. Forexample, a ground plane layer, such as layer 208, may be provided on thesecond principal surface of the elongated flexible substrate to providea ground plane, and multiple strips, such as strips 308 may be formed onthe surface of the ground plane layer to provide mechanical stability.For instance, the ground plane layer may be formed from niobium, whereasthe strips formed on the surface of the ground plane layer may be formedfrom copper. The dimensions described herein with respect to layer 208may also be applied to the ground plane layer. Similarly, the dimensionsand spacing described herein with respect to strips 308 may also beapplied to the strips formed on the ground plane layer.

In some implementations, the number of wires contained within a flexiblewiring can be increased by stacking the flexible wiring. For example,any of the flexible wiring 200, 250 or 300 may be stacked together toprovide stacked flexible wiring. In some cases, the flexible wiring maybe stacked together using an adhesive-based or adhesiveless laminatebonding technique (e.g., the application of heat and/or pressure to bondpolyimide layer together). Use of adhesiveless polyimide bonding can beadvantageous in certain implementations, as it eliminates the adhesivethat sometimes leads to outgassing under vacuum environments.Furthermore, adhesiveless laminates may have a coefficient of thermalexpansion (CTE) that is closely matched to the CTE of copper, thusreducing stress between the substrate and shielding/traces caused bysubstantial changes in temperature when cooling down to extremely lowtemperatures, such as the temperatures used in a cryostat. In somecases, the stacked flexible wiring can be formed using polymerencapsulants that are sprayed or painted on an initial elongated polymersubstrate that includes the electrically conductive traces/shieldinglayer. For instance, in some cases, a flexible wiring such as wiring200, 250 or 300 can be sprayed or painted with an epoxy encapsulant(e.g., Stycast 2850FT) that is then cured to provide an additionalpolymer layer on which further electrically conductive materials may bedeposited and patterned.

In contrast to the flexible wiring illustrated in FIGS. 2-4, in someimplementations, the flexible wiring may be formed as a stripline. FIG.5 is a schematic that illustrates an example of flexible wiring 500formed in a stripline configuration. The flexible wiring 500 includes asignal trace 506 arranged between a first elongated flexible substrateportion 502 and a second elongated flexible substrate portion 504. Thesignal trace 506 may include an electrically conductive thin filmmaterial for transmitting control and/or data signals. A top surface(e.g., a first principal surface) of the first elongated flexiblesubstrate portion 502 may include a first electrically conductive layer508, whereas a bottom surface (e.g., a second principal surface) of thesecond elongated flexible substrate portion 504 may include a secondelectrically conductive layer 510. The first electrically conductivelayer 508, the second electrically conductive layer 510, and the signaltrace 506 may include, e.g., a thin film materials such as a metal orsuperconductor thin films. For instance, the metal or superconductorthin film may include a copper, copper alloy, aluminum, niobium, orindium thin film layer. Although only a single signal trace 506 isillustrated in FIG. 5, multiple signal traces 506 may be includedbetween the first elongated flexible substrate portion 502 and thesecond elongated flexible substrate portion 504 (e.g., along the Y-axisinto and out of the page of FIG. 5). For example, such signal traces 506may be aligned in parallel in a similar manner to electricallyconductive traces 204 shown in FIG. 2.

In some cases, the first electrically conductive layer 508, the secondelectrically conductive layer 510, and/or the signal trace 506 mayinclude a bi-layer film, such as described herein with respect toflexible wiring 200. For example, as shown in FIG. 5, the secondelectrically conductive layer 510 may include a bi-layer film having afirst thin film layer 518 formed on and/or in contact with the bottomsurface of the second elongated flexible substrate portion 504. Thebi-layer film further may include a second thin film layer 516 formed onand in contact with the first thin film layer 518. The bi-layer filmalso may be formed on the top surface of the first elongated flexiblesubstrate portion 502. In some implementations, portions of the secondlayer 516 are removed, revealing the underlying first layer 518.

The length of the signal traces 506 may be as long as the length of theelongated flexible substrate portions 502, 504. The width of each signaltrace 506 may be, e.g., between about 1 μm to about 250 μm, includingwidths such as 5 μm, 10 μm, 20 μm, 30 μm, 50 μm, or 100 μm, amongothers. The thickness of each signal trace 506 may be, e.g., betweenabout 10 nm to about 100 μm, including thicknesses such as 50 nm, 100nm, 250 nm, 500 nm, 750 nm, 1 μm, 5 μm, 10 μm, 20 μm, 50 μm, amongothers. In the case of bi-layer conductive traces, each layer may havethe same or different thicknesses. For example, in some implementations,the first layer has a thickness of 2 μm, whereas the second layer has athickness of 5 μm. Alternatively, in some cases, the first layer has athickness of 20 μm, whereas the second layer has a thickness of 5 μm.The conductive traces 204 may be separated by a constant or variablepitch. For example, in some implementations, the pitch between adjacentsignal traces 506 is between about 1 μm to about 1 mm, including pitchessuch as 5 μm, 10 μm, 50 μm, 100 μm, 250 μm, 500 μm, or 750 μm, amongothers. The signal traces 506 may be formed on the elongated flexiblesubstrate portion 502 or portion 504 using integrated chip (IC)fabrication techniques such as deposition (e.g., sputtering and vapordeposition), etching, and/or lift-off techniques.

Each of the first elongated flexible substrate portion 502 and thesecond elongated flexible substrate portion 504 may be formed, e.g.,from a flexible plastic ribbon, such as a polyimide ribbon (e.g., poly(4,4′-oxydiphenylene-pyromellitimide)). The first elongated flexiblesubstrate portion 502 may be bonded to the second elongated flexiblesubstrate portion 504. The thickness of the substrate portions 502 and504 may be, e.g., between about 10 μm to about 500 μm, includingthicknesses such as 20 μm, 50 μm, 75 μm, and 100 μm, among others. Thewidth of the elongated flexible substrate 202 may be, e.g., betweenabout 1 mm to about 30 mm, including widths such as 10 mm, 15 mm, and 20mm, among others. The length of the elongated flexible substrateportions 502 and 504 may be at least as long as is necessary to providecoupling between devices, systems and/or other wiring.

The first electrically conductive layer 508 and the second electricallyconductive layer 510 may each correspond to electromagnetic shieldinglayers that shield the signal trace traces 506 from external signalnoise. In some implementations, the flexible wiring 500 can be stackedtogether with one or more flexible wiring 500 to provide a stackedflexible wiring having an increase number of signal lines fortransmitting control and/or data signals. As explained herein, theflexible wiring may be stacked together using an adhesive-based oradhesiveless laminate bonding technique. In some cases, the stackedflexible wiring can be formed using polymer encapsulants that aresprayed or painted on an initial elongated polymer substrate thatincludes the electrically conductive traces/shielding layer. Forinstance, in some cases, a flexible wiring such as wiring 500 can besprayed or painted with an epoxy encapsulant (e.g., Stycast 2850FT) thatis then cured to provide an additional polymer layer on which furtherelectrically conductive materials may be deposited and patterned.

In some implementations, the flexible wiring 500 includes one or morevias 512 extending through the first elongated flexible substrateportion 502. The vias 512 may include an electrically conductivematerial (via contact 514) formed within the vias 512. The via contact514 may include, e.g., a superconducting and/or non-superconductingmetal, such as copper, aluminum, niobium, indium, or a copper alloy. Insome cases, the via contact 514 is formed on the sidewalls of the via512 but does not completely fill the via 512. In other cases, the viacontact 514 completely fills the via 512, such that there is nocontinuous opening that extends through the via 512. In someimplementations, the via contact 514 includes an adhesive layer formedto help the via contact 514 adhere to the sides of the via 512. Forinstance, the adhesive layer may include a film (e.g., between about 1nm to about 1 micron thick, defined relative to the via sidewall) ofcopper or niobium. In some implementations, the material of the viacontact 514 that is formed on the adhesive layer may include a film(e.g., between about 500 nm to about 20 micron thick, defined relativeto the via sidewall) of copper or niobium.

The adhesive layer may be formed, e.g., by using electroless plating ofthe adhesive layer material (e.g., Cu) to establish a thin firstadhesive layer, and then performing electroplating of the adhesive layermaterial (e.g., Cu) to establish a second adhesive layer on the firstadhesive layer. Then, the remaining material (e.g., Al, Cu, or Nb) ofthe via contact 514 also may be formed on the adhesive layer using,e.g., electroplating. For example, aluminum may be plated (e.g.,electroplated) on the adhesive layer. Other plating techniques may beused to plate the via contact 514 as well. For example, solvent basedplating may be used to form a niobium via contact.

In some implementations, the via 512 extends from the first electricallyconductive layer 508 to the signal trace 506 so that the via contact 514connects the first electrically conductive layer 508 to the signal trace506. In some implementations, the via 512 extends from the firstelectrically conductive layer 508 to the second electrically conductivelayer 510 so that the via contact 514 connects the first electricallyconductive layer 508 to the second electrically conductive layer 510. Insome implementations, the via 512 extends from the second electricallyconductive layer 510 to the signal trace 506 so that the via contact 514connects the second electrically conductive layer 510 to the signaltrace 506. The via 512 may be formed using laser drill technology.

In some implementations, the flexible wiring 500 has regions in whichthe signal trace 506 is exposed so that an electrical connection, suchas a wire bond or bump bond may be made to the signal trace. Exposingthe signal trace 506 may include removing parts of the first elongatedflexible substrate portion 502 and/or removing parts of the secondelongated flexible substrate portion 504 that cover the signal trace506. In some cases, the length of the first elongated flexible substrateportion 502 and/or the second elongated flexible substrate portion 504are not long enough to cover the entirety of the signal trace 506 suchthat a portion of the signal trace 506 is exposed.

An example of a technique for connecting flexible wiring to one another,to the quantum information processing system, and/or to circuitcomponents is to use coaxial connectors, such as SMA connectors.However, coaxial connects can be bulky and therefore take up a lot oflimited available space within the cryostat. Furthermore, the bulkinessof the coaxial connector also can render difficult making connections tothe contacts of the high density flexible wiring. An alternative tocoaxial connectors is to use a modified butt joint bond that employswire bonding between contacts of the flexible wiring. FIG. 6 is aschematic that illustrates an example of a modified butt joint bond thatemploys wire bonding. In particular, FIG. 6 depicts a cross-section of afirst flexible wiring 602 that is coupled to a second flexible wiring604 using a modified butt joint bond. Each of first flexible wiring 602and second flexible wiring 604 may have the same configuration as theflexible wiring 200 shown in FIG. 2. For example, flexible wiring 602may include an elongated flexible substrate 606, multiple electricallyconductive traces 608 (one trace 608 is shown in FIG. 6) arranged on aprincipal surface of the elongated flexible substrate 606, and anelectrically conductive layer 610 on a second principal surface of theelongated flexible substrate 606. Similarly, flexible wiring 604 mayinclude an elongated flexible substrate 612, multiple electricallyconductive traces 614 (one trace 614 is shown in FIG. 6) arranged on aprincipal surface of the elongated flexible substrate 616, and anelectrically conductive layer 616 on a second principal surface of theelongated flexible substrate 616. As in the flexible wiring 200 shown inFIG. 2, each conductive trace 608, 614 corresponds to an individual wireand multiple traces may be arranged in an array (e.g., along theY-direction into and out of the page of FIG. 6). Additionally, theelectrically conductive layers 610 and 616 can be electromagneticshielding layers for shielding the conductive traces 608, 614,respectively, from crosstalk. Though both flexible wiring 602 and 604are shown flat in FIG. 6, they can include fold regions, such as inflexible wiring 200, so that the layers 610, 616 can provide shieldingfor traces 608, 614.

First flexible wiring 602 is arranged with an edge 601 facing an edge603 of second flexible wiring 604. Edge 601 may be separated from edge603 by a relatively small distance 622 or touching one another. Forexample, distance 622 may be between about 25 microns to about severalmillimeters, such as 100 μm or 250 μm, among other distances. Wire bonds618 are provided that may be used to electrically connect traces 608 offirst flexible wiring 602 to traces 614 of second flexible wiring 604.

In some implementations, a solder bridge may be used instead of wirebonds to electrically connect traces 608 of first flexible wiring 602 totraces 614 of second flexible wiring 604. The distance 622 should bekept as small as possible to allow the solder bridge to form. Solderused to form the wire bonds 618 or solder bridge may be formed from asuperconducting or non-superconducting material.

Both the edge 601 of the first flexible wiring 602 and the edge 603 ofthe second flexible wiring 604 may be cut using laser processing toprovide more precise and relatively smooth edges. The edges 601 and 603then may be placed closer together to provide a smaller bridge lengthfor the solder bridge, which improves connection integrity andfacilitates the bonding process.

In some implementations, the joint between the first flexible wiring 602and the second flexible wiring 604 is fixed against a metal block toprovide a mechanical connection for the first and second flexible wiring602, 604, to provide an electrical connection between the first andsecond flexible wiring 602, 604, and/or to maintain the wiring at thetemperature of the cryostat stage in which the wiring is arranged. Forexample, as shown in FIG. 6, a metal block 620 may be secured to and inthermal contact with the electromagnetic shielding layers 610, 616. Insome implementations, the metal block 620 is clamped in place againstthe flexible wiring 602 and 604. Alternatively, or in addition, themetal block 620 is fixed to the shielding layers 610, 616 through anadhesive, such as solder. The metal block 620 may be formed from amaterial suitable for providing sufficient heat transfer within thecryostat, such as copper. In some implementations, the shielding layers610, 616 double as grounding planes, and the metal block 620 provides acommon ground.

FIG. 6 illustrates a modified butt joint for flexible wiring having theconfiguration provided as shown in FIG. 2. In some implementations, amodified butt joint also may be used for flexible wiring having theconfiguration shown in FIG. 5. For example, FIG. 7A is a schematicillustrating a cross-section of a first flexible wiring 702 that iscoupled to a second flexible wiring 704 using the modified butt joint.Each of the first flexible wiring 702 and the second flexible wiring 704has the same stripline configuration as the flexible wiring 500 shown inFIG. 5. For example, flexible wiring 702 may include a first elongatedflexible substrate portion 706, a second elongated flexible substrateportion 708, a signal trace 714 arranged between portions 706 and 708, afirst electrically conductive layer 710 on a top surface of substrateportion 706, and a second electrically conductive layer 712 on a bottomsurface of substrate portion 708. Similarly, flexible wiring 704 mayinclude a first elongated flexible substrate portion 716, a secondelongated flexible substrate portion 718, a signal trace 724 arrangedbetween portions 716 and 718, a first electrically conductive layer 720on a top surface of substrate portion 716, and a second electricallyconductive layer 722 on a bottom surface of substrate portion 718. As inthe flexible wiring 500 shown in FIG. 5, each signal trace 714, 724corresponds to an individual wire and multiple traces may be arranged inan array (e.g., along the Y-direction into and out of the page of FIG.7A). Additionally, the electrically conductive layers 710, 712, 720, 722can be electromagnetic shielding layers for shielding the signal traces714, 724 from crosstalk.

First flexible wiring 702 is arranged with an edge 701 facing an edge703 of second flexible wiring 704. Edge 701 may be separated from edge703 by a relatively small distance or touching one another. For example,edge 701 may be separated from edge 703 by a distance between about 25microns to about several millimeters, such as 100 μm or 250 μm amongother distances. Wire bonds 730 are provided that may be used toelectrically connect traces 714 of first flexible wiring 702 to traces724 of second flexible wiring 704. In some implementations, a solderbridge may be used instead of wire bonds to connect traces 714 of firstflexible wiring 702 to traces 724 of second flexible wiring 704. Thedistance between the first edge 701 and the second edge 703 should bekept as small as possible to allow the solder bridge to form. Both theedge 701 of the first flexible wiring 702 and the edge 703 of the secondflexible wiring 704 may be cut using laser processing to provide moreprecise and relatively smooth edges. The edges 701 and 703 then may beplaced closer together to provide a smaller bridge length for the solderbridge, which improves connection integrity and facilitates the bondingprocess. Solder used to form the wire bonds 730 or solder bridge may beformed from a superconducting or non-superconducting material.

In some implementations, first flexible wiring 702 may include a region726 in which the substrate portion 706 is removed or is absent to exposea section of signal trace 714. Similarly, second flexible wiring 704 mayinclude a region 728 in which the substrate portion 716 is removed or isabsent to expose a section of signal trace 724. By exposing the signaltraces in regions 726, 728, the signal traces 726, 728 can be accessedto form the wired bond or solder bridge bond. FIG. 7B is a schematicthat illustrates a side view of edge 703 and region 728 of secondflexible wiring 704 from FIG. 7A. As shown in FIG. 7B, only a section ofsubstrate portion 716 directly above signal trace needs to be removed orabsent to form region 728 and expose the signal trace 724. Substrateportion 716 to the left and to the right of the region 728 may be leftin place to bond to substrate portion 718 and to provide a supportsurface for conductive layer 720. In some implementations, regions 726,728 include the solder that forms the electrical connection between thefirst flexible wiring 702 and the second flexible wiring 704.

In some implementations, the joint between the first flexible wiring 702and the second flexible wiring 704 is secured to and in thermal contactwith a metal block to provide a mechanical connection between first andsecond flexible wiring 702, 704, to provide an electrical connectionbetween first and second flexible wiring 702, 704, and/or to maintainthe wiring at the temperature of the cryostat stage in which the wiringis arranged. For example, as shown in FIG. 7A, a metal block 732 may bepositioned against electromagnetic shielding layers 712, 722. In someimplementations, the metal block 732 is clamped in place against theflexible wiring 702 and 704. Alternatively, or in addition, the metalblock 732 is fixed to the shielding layers 712, 722 through an adhesive,such as solder. Alternatively, or in addition, an additional metal blockis secured to and in thermal contact with the shielding layers 710, 720.The additional metal block also may be fixed to shielding layers 710,720 through an adhesive, such as solder. The metal blocks may be formedfrom a material suitable for providing sufficient heat transfer withinthe cryostat, such as copper. In some implementations, the shieldinglayers 712, 722 double as grounding planes, and the metal block 732provides a common ground. Similarly, the shielding layers 710, 720 maydouble as grounding planes to which the additional metal block providesa common ground.

In some implementations, the joint between a first flexible wiring and asecond flexible wiring may be provided at a boundary within a cryostatthat separates one temperature stage of the cryostat held at a firsttemperature and a second temperature stage of the cryostat held at asecond different temperature from the first stage. For example, thejoint may connect a first flexible wiring, such as first flexible wiring602 or 702, within a temperature stage held to a temperature below 3 K(e.g., stage 103 in FIG. 1), to a second flexible wiring, such as secondflexible wiring 604 or 704, within a temperature stage held to atemperature above 3 K but below room temperature (e.g., stage 101 inFIG. 1). In some implementations, the flexible wiring at a transitionbetween different temperature stages within a cryostat or at atransition from a vacuum environment to another vacuum environment or toa non-vacuum environment may be sealed at the transition using an epoxyadhesive fixed to the flexible wiring and to a clamp device (e.g., ametal ring such as a copper ring).

Various approaches may be used to fabricate the flexible wiring asdisclosed herein. For example, in some implementations, the flexiblewiring may be constructed by providing a large substrate (e.g., aflexible plastic substrate, such as polyimide) on which the metalsand/or superconducting films are formed. The substrate may include, forexample, a large sheet that is greater than 8″ on a side, e.g., 12″ by14″. For depositing a metal/superconducting film, the substrate may beplaced in a vacuum chamber. Prior to depositing any films, the substratesurface may be cleaned, e.g., by performing an ion cleaning (e.g., an Arion cleaning). In the case a bi-layer film is formed on the substrate, afirst layer of material is blanket deposited on the substrate. The firstlayer may include, e.g., a superconductor film, such as niobium that isdeposited using sputtering. Alternatively, the first layer may include anon-superconducting film such as copper. The first layer may bedeposited to have a thickness up to about 5 μm. For example, the firstlayer may be deposited to have a thickness of 100 nm, 250 nm, 500 nm,750 nm, 1 μm, or 2 μm, among other thicknesses. A second layer then isblanket deposited (e.g., sputtered or electroless plating) on the firstlayer. The second layer may include a non-superconducting film, such ascopper, or a superconducting film such as niobium or aluminum. Thesecond layer may be deposited to have a thickness up to about 20 μm. Forexample, the second layer may be deposited to have a thickness of 100nm, 250 nm, 500 nm, 750 nm or 1 μm. In some cases, a first depositedportion of the second layer serves as a base layer for a laterelectroplating step. For example, a thin 100 nm film of copper may bedeposited after which a thicker layer of copper is electroplated. Insome implementations, the films are deposited on both a top and bottomside of the substrate. The deposited films then may be patterned (e.g.,using etching or lift-off processes) to form the desired circuitpattern. In some cases for a bi-layer film, an identical pattern istransferred to both the first layer and the second layer during thepatterning step. In other cases, a different pattern is formed for thefirst layer then for the second layer in the patterning step. In someimplementations, via holes are formed within the substrate using a laseretch process. The via holes may then be filled with via contact material(e.g., copper and/or a superconducting material) to form the viacontacts. Once patterned, the substrate sheet may be partitioned intoindividual flexible wiring. Partitioning the substrate sheet may entailperforming laser cutting on the substrate sheet or using a blade tomechanically cut the substrate sheet. In some implementations,partitioning the substrate sheet results in a finalized flexible wiring.Alternatively, in some implementations, the partitioned substrate sheetsmay be stacked together to form a stacked flexible wiring (e.g., a stackof multiple flexible wiring 200), to form a stripline configuration(e.g., flexible wiring 500), or to form a stacked stripline flexiblewiring. Stacking the partitioned substrate sheets may entail introducingan adhesive between the substrate that is cured so that the stackedsubstrates are bonded together. Alternatively, the partitioned substratesheets may be bonded together using adhesiveless bonding techniques.After obtaining the stacked flexible wiring, further processing may beperformed if necessary. For example, additional via contacts may beformed within one or more of the stacked substrates to provideconnections to conductive traces on the stacked flexible wiring.

In some implementations, the flexible wiring may be constructed using anextrusion and roll process. For example, a first elongated sheet ofsuperconducting or non-superconducting material (e.g., a 0.25″ thicksheet of niobium, aluminum or copper) may be provided. In some cases, asecond elongated sheet of superconducting or non-superconductingmaterial (e.g., a 0.25″ thick sheet of niobium, aluminum or copper) maybe provided on top of the first elongated sheet. If just a single sheetis provided, the single sheet of material is passed through an extrusionmachine that thins the sheet (e.g., to a thickness of between about 20microns to about 10 mm). If a bi-layer is provided, the first and secondelongated sheets then may be pressed together under vacuum and/or heatand passed through the extrusion machine that thins the bi-layer sheet(e.g., to a thickness of between about 20 microns to about 10 mm) andcauses the materials in the bi-layer sheet to adhere together. Thethinned single layer or bi-layer sheet then may be laminated with apolyimide substrate. In some implementations, a thinned single layer orbi-layer sheet is laminated on both sides of the polyimide substrate. Asexplained herein, superconducting and/or non-superconducting films onthe polyimide substrate may be patterned (e.g., using etching processes)to form the desired circuit pattern. In some cases, for a bi-layer film,an identical pattern is transferred to both the first layer and thesecond layer during the patterning step. In other cases, a differentpattern is formed for the first layer then for the second layer in thepatterning step. In some implementations, via holes are formed withinthe substrate using a laser etch process. The via holes may then befilled with via contact material (e.g., copper and/or a superconductingmaterial) to form the via contacts. Once patterned, the substrate sheetincluding the patterned films may be partitioned into individualflexible wiring. Partitioning the substrate sheet may entail performinglaser cutting on the substrate sheet or using a blade to mechanicallycut the substrate sheet. In some implementations, partitioning thesubstrate sheet results in a finalized flexible wiring. Alternatively,in some implementations, the partitioned substrate sheets may be stackedtogether to form a stacked flexible wiring (e.g., a stack of multipleflexible wiring 200), to form a stripline configuration (e.g., flexiblewiring 500), or to form a stacked stripline flexible wiring. Stackingthe partitioned substrate sheets may entail introducing an adhesivebetween the substrate that is cured so that the stacked substrates arebonded together. Alternatively, the partitioned substrate sheets may bebonded together using adhesiveless bonding techniques. After obtainingthe stacked flexible wiring, further processing may be performed ifnecessary. For example, addition via contact may be formed within one ormore of the stacked substrates to provide connections to conductivetraces on the stacked flexible wiring.

Implementations of the quantum subject matter and quantum operationsdescribed in this specification can be implemented in suitable quantumcircuitry or, more generally, quantum computational systems, alsoreferred to as quantum information processing systems, including thestructures disclosed in this specification and their structuralequivalents, or in combinations of one or more of them. The terms“quantum computational systems” and “quantum information processingsystems” may include, but are not limited to, quantum computers, quantumcryptography systems, topological quantum computers, or quantumsimulators.

The terms quantum information and quantum data refer to information ordata that is carried by, held or stored in quantum systems, where thesmallest non-trivial system is a qubit, e.g., a system that defines theunit of quantum information. It is understood that the term “qubit”encompasses all quantum systems that may be suitably approximated as atwo-level system in the corresponding context. Such quantum systems mayinclude multi-level systems, e.g., with two or more levels. By way ofexample, such systems can include atoms, electrons, photons, ions orsuperconducting qubits. In some implementations the computational basisstates are identified with the ground and first excited states, howeverit is understood that other setups where the computational states areidentified with higher level excited states are possible. It isunderstood that quantum memories are devices that can store quantum datafor a long time with high fidelity and efficiency, e.g., light-matterinterfaces where light is used for transmission and matter for storingand preserving the quantum features of quantum data such assuperposition or quantum coherence.

Quantum circuit elements (also referred to as quantum computing circuitelements) include circuit elements for performing quantum processingoperations. That is, the quantum circuit elements are configured to makeuse of quantum-mechanical phenomena, such as superposition andentanglement, to perform operations on data in a non-deterministicmanner. Certain quantum circuit elements, such as qubits, can beconfigured to represent and operate on information in more than onestate simultaneously. Examples of superconducting quantum circuitelements include circuit elements such as quantum LC oscillators, qubits(e.g., flux qubits, phase qubits, or charge qubits), and superconductingquantum interference devices (SQUIDs) (e.g., RF-SQUID or DC-SQUID),among others.

In contrast, classical circuit elements generally process data in adeterministic manner. Classical circuit elements can be configured tocollectively carry out instructions of a computer program by performingbasic arithmetical, logical, and/or input/output operations on data, inwhich the data is represented in analog or digital form. In someimplementations, classical circuit elements can be used to transmit datato and/or receive data from the quantum circuit elements throughelectrical or electromagnetic connections. Examples of classical circuitelements include circuit elements based on CMOS circuitry, rapid singleflux quantum (RSFQ) devices, reciprocal quantum logic (RQL) devices andERSFQ devices, which are an energy-efficient version of RSFQ that doesnot use bias resistors.

Fabrication of the quantum circuit elements and classical circuitelements described herein can entail the deposition of one or morematerials, such as superconductors, dielectrics and/or metals. Dependingon the selected material, these materials can be deposited usingdeposition processes such as chemical vapor deposition, physical vapordeposition (e.g., evaporation or sputtering), or epitaxial techniques,among other deposition processes. Processes for fabricating circuitelements described herein can entail the removal of one or morematerials from a device during fabrication. Depending on the material tobe removed, the removal process can include, e.g., wet etchingtechniques, dry etching techniques, or lift-off processes. The materialsforming the circuit elements described herein can be patterned usingknown lithographic techniques (e.g., photolithography or e-beamlithography).

During operation of a quantum computational system that usessuperconducting quantum circuit elements and/or superconductingclassical circuit elements, such as the circuit elements describedherein, the superconducting circuit elements are cooled down within acryostat to temperatures that allow a superconductor material to exhibitsuperconducting properties. A superconductor (alternativelysuperconducting) material can be understood as material that exhibitssuperconducting properties at or below a superconducting criticaltemperature. Examples of superconducting material include aluminum(superconductive critical temperature of about 1.2 kelvin), indium(superconducting critical temperature of about 3.4 kelvin), NbTi(superconducting critical temperature of about 10 kelvin) and niobium(superconducting critical temperature of about 9.3 kelvin). Accordingly,superconducting structures, such as superconducting traces andsuperconducting ground planes, are formed from material that exhibitssuperconducting properties at or below a superconducting criticaltemperature.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of what may beclaimed, but rather as descriptions of features that may be specific toparticular implementations. Certain features that are described in thisspecification in the context of separate implementations can also beimplemented in combination in a single implementation. Conversely,various features that are described in the context of a singleimplementation can also be implemented in multiple implementationsseparately or in any suitable sub-combination. Moreover, althoughfeatures may be described above as acting in certain combinations andeven initially claimed as such, one or more features from a claimedcombination can in some cases be excised from the combination, and theclaimed combination may be directed to a sub-combination or variation ofa sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. For example, the actions recited in the claims can be performedin a different order and still achieve desirable results. In certaincircumstances, multitasking and parallel processing may be advantageous.Moreover, the separation of various components in the implementationsdescribed above should not be understood as requiring such separation inall implementations.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the invention. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. Flexible wiring comprising: an elongated flexiblesubstrate; a plurality of electrically conductive traces arranged in anarray on a first side of the elongated flexible substrate; and anelectromagnetic shielding layer on a second side of the elongatedflexible substrate, the second side being opposite the first side,wherein the elongated flexible substrate comprises a fold region betweena first electronically conductive trace and a second electricallyconductive trace such that the electromagnetic shielding layer provideselectromagnetic shielding between the first electronically conductivetrace and the second electrically conductive trace.
 2. The flexiblewiring of claim 1, wherein the fold region comprises a raised band inthe flexible substrate, and a length of the elongated raised bandextends parallel to a length of the first electrically conductive traceand the second electrically conductive trace.
 3. The flexible wiring ofclaim 1, comprising a first elongated groove in the fold region, and alength of the first elongated groove runs parallel to a length of thefirst electrically conductive trace and a length of the secondelectrically conductive trace.
 4. The flexible wiring of claim 3,wherein the first elongated groove extends into the first side or intothe second side of the elongated flexible substrate.
 5. The flexiblewiring of claim 3, comprising a second elongated groove in the foldregion, a length of the second elongated groove runs parallel to thelength of the first electrically conductive trace and the length of thesecond electrically conductive trace, and the first elongated groove ison the first side of the substrate and the second elongated groove is onthe second side of the substrate.
 6. The flexible wiring of claim 3,wherein the first elongated groove extends into the electromagneticshielding layer.
 7. The flexible wiring of claim 6, wherein the firstelongated groove extends into the elongated flexible substrate.
 8. Theflexible wiring of claim 1, wherein at least one electrically conductivetrace of the plurality of electrically conductive traces comprises abi-layer, the bi-layer comprising a superconductor layer and a metallayer on the superconductor layer.
 9. The flexible wiring of claim 8,wherein the superconductor layer comprises niobium or NbTi.
 10. Theflexible wiring of claim 8, wherein the metal layer comprises copper ora copper alloy.
 11. The flexible wiring of claim 1, wherein theelectromagnetic shielding layer comprises a bi-layer, the bi-layercomprising a superconductor layer and a metal layer on thesuperconductor layer.
 12. The flexible wiring of claim 11, wherein thesuperconductor layer comprises niobium.
 13. The flexible wiring of claim11, wherein the metal layer comprises copper or a copper alloy.
 14. Theflexible wiring of claim 1, wherein the electromagnetic shielding layercomprises a plurality of microstrips having lengths orientedorthogonally with respect to the lengths of the plurality ofelectrically conductive traces.
 15. A flexible wiring comprising: afirst elongated flexible layer; a second elongated flexible layer bondedto the first elongated flexible layer; a plurality of electricallyconductive traces arranged at a bond interface between the firstelongated flexible layer and the second elongated flexible layer; afirst electromagnetic shielding layer on a principal surface of thefirst elongated flexible layer; a second electromagnetic shielding layeron a principal surface of the second elongated flexible layer; and a viaextending through the first elongated flexible layer, wherein the viacomprises a superconductor via contact.
 16. The flexible wiring of claim15, wherein the via comprises an adhesive layer, the superconductor viacontact being formed on the adhesive layer.
 17. The flexible wiring ofclaim 15, wherein the via extends from the first electromagneticshielding layer to at least one electrically conductive trace of theplurality of electrically conductive traces, and the superconductor viacontact is connected to the first electromagnetic shielding layer andthe at least one electrically conductive trace.
 18. The flexible wiringof claim 15, wherein the via extends from the first electromagneticshielding layer to the second electromagnetic shielding layer, and thesuperconductor via contact is connected to the first electromagneticshielding layer and the at least one electrically conductive trace. 19.A device comprising: a first flexible wiring comprising a firstelongated flexible substrate, a first plurality of electricallyconductive traces arranged in an array on a first side of the firstelongated flexible substrate, and a first electromagnetic shieldinglayer on a second side of the first elongated flexible substrate, thesecond side of the first elongated flexible substrate being opposite thefirst side of the first elongated flexible substrate; a second flexiblewiring comprising a second elongated flexible substrate, a secondplurality of electrically conductive traces arranged in an array on afirst side of the second elongated flexible substrate, and a secondelectromagnetic shielding layer on a second side of the second elongatedflexible substrate, the second side of the second elongated flexiblesubstrate being opposite the first side of the second elongated flexiblesubstrate, wherein the first flexible wiring is coupled to the secondflexible wiring through a butt joint.
 20. The device of claim 19,wherein the butt joint comprises a wire bond that connects a firstelectrically conductive trace from the first plurality of electricallyconductive traces to a first electrically conductive trace from thesecond plurality of electrically conductive traces.
 21. The device ofclaim 19, wherein the butt joint comprises a solder bridge that connectsa first electrically conductive trace from the first plurality ofelectrically conductive traces to a first electrically conductive tracefrom the second plurality of electrically conductive traces.
 22. Thedevice of claim 19 further comprising a metal block secured to and inthermal contact with the first electromagnetic shielding layer and tothe second electromagnetic shielding layer.
 23. A device comprising: afirst flexible wiring comprising a first elongated flexible substrate, afirst plurality of electrically conductive traces arranged at a bondinterface within the first elongated flexible substrate, a firstelectromagnetic shielding layer on a first principal surface of thefirst elongated flexible substrate, and a second electromagneticshielding layer on a second principal surface of the first elongatedflexible substrate; a second flexible wiring comprising a secondelongated flexible substrate, a second plurality of electricallyconductive traces arranged at a bond interface within the secondelongated flexible substrate, a third electromagnetic shielding layer ona first principal surface of the second elongated flexible substrate,and a fourth electromagnetic shielding layer on a second principalsurface of the second elongated flexible substrate, wherein the firstflexible wiring is electrically coupled to the second flexible wiringthrough a butt joint.
 24. The device of claim 23, wherein the firstelongated flexible substrate comprises a first cavity through which afirst electrically conductive trace of the first plurality ofelectrically conductive traces is exposed, and the second elongatedflexible substrate comprises a second cavity through a firstelectrically conductive trace of the second plurality of electricallyconductive traces is exposed.
 25. The device of claim 24, wherein thebutt joint comprises a wire bond that connects the exposed firstelectrically conductive trace of the first plurality of electricallyconductive traces to the exposed first electrically conductive trace ofthe second plurality of electrically conductive traces
 26. The device ofclaim 24, wherein the butt joint comprises a solder bridge that connectsthe exposed first electrically conductive trace of the first pluralityof electrically conductive traces to the exposed first electricallyconductive trace of the second plurality of electrically conductivetraces.
 27. The device of claim 24, further comprising a first metalblock secured to and in thermal contact with the first electromagneticshielding layer and to the third electromagnetic shielding layer. 28.The device of claim 27, further comprising a second metal block securedto and in thermal contact with the second electromagnetic shieldinglayer and to the fourth electromagnetic shielding layer.
 29. A systemcomprising: a cryostat comprising a first stage configured to be heldwithin a first temperature range; a quantum information processingsystem within the first stage; and flexible wiring within the firststage and coupled to the quantum information processing system, whereinthe flexible wiring comprises an elongated flexible substrate, aplurality of electrically conductive traces arranged in an array on afirst side of the elongated flexible substrate, and an electromagneticshielding layer on a second side of the elongated flexible substrate,the second side being opposite the first side, wherein the elongatedflexible substrate comprises a fold region between a firstelectronically conductive trace and a second electrically conductivetrace such that the electromagnetic shielding layer provideselectromagnetic shielding between the first electronically conductivetrace and the second electrically conductive trace
 30. A systemcomprising: a cryostat comprising a first stage configured to be heldwithin a first temperature range; a quantum information processingsystem within the first stage; and flexible wiring within the firststage and coupled to the quantum information processing system, whereinthe flexible wiring comprises a first elongated flexible layer; a secondelongated flexible layer bonded to the first elongated flexible layer; aplurality of electrically conductive traces arranged at a bond interfacebetween the first elongated flexible layer and the second elongatedflexible layer; a first electromagnetic shielding layer on a principalsurface of the first elongated flexible layer; a second electromagneticshielding layer on a principal surface of the second elongated flexiblelayer; and a via extending through the first elongated flexible layer,wherein the via comprises a superconductor via contact.